Thin film optical recording layers using chalcogenide thin films and amorphous to crystalline phase transitions have been the subject of many investigations since the early 1970's. The initial interests were focused on "erasable", and therefore reusable, optical recording layers since the amorphous to crystalline transition is, in principle, a reversible process. A low power, relatively long duration laser pulse is used to heat a local spot on the layer to below the melting point for a sufficient length of time to cause the spots to crystallize. These crystalline spots can in turn be heated, by a higher power, shorter duration laser, above the melting point of the crystallized spots to randomize the structure of the spots. The layer is designed such that upon the termination of the laser pulse, the cooling rate of the heated spot is high enough that the randomized structure is frozen to achieve an amorphous state.
Thus, by adjusting the laser power and duration, the state of a selected area on the layer can be switched between the amorphous state and the crystalline state to create a pattern of amorphous and crystalline spots which can be used for information storage. Since the phase transition is reversible, &he pattern can be erased and replaced with a different recorded pattern. Theoretically, this erase write cycle can be carried out any number of times.
Very few materials are known for optical recording layers in which the above described write-erase-write cycle is of practical use. No erasable phase-change type optical recording layers have been commercialized.
European Patent Application No. 0184452 discloses certain erasable optical recording layers of antimony indium and antimony-indium-tin alloys. Information recording and erasure are said to be achieved by switching the layers between two different crystalline states. The layers are generally prepared in the amorphous state which has to be first converted into one of the two crystalline states before information can be recorded. The crystallized states, achieved by either a bulk heat-treatment or a prolonged laser exposure, are said to have a lower reflectance than the amorphous state. The examples indicate that the materials disclosed therein have a very slow rate of crystallization. This application further teaches that the optical recording layers disclosed therein are unsuitable for use in the amorphous-to-crystalline transition mechanism because of the instability of the amorphous state in general. Thus, because of the slow amorphous to crystalline transition and the instability of the amorphous state, the alloys disclosed in this reference are not suited to write-once recording.
A good deal of attention has also focused on so-called "write once" thin film optical recording layers. Write-once simply means that the layers can be recorded upon only once. Such layers cannot be erased and reused for a subsequent recording.
Since thin film optical recording layers are generally amorphous when prepared, it is desirable to use the crystallization step as the recording step in write-once layers. However, the problem of slow crystallization prevents the achievement of high data rates with most known materials. High data rates are critical for write-once layers designed for use with computers.
Thus, a principal difficulty is that the rate of crystallization of most layers studied is usually too low. For practical applications, it is desirable to have layers which can be crystallized by laser pulses shorter than a microsecond (.mu.s). Presently, few materials have demonstrated such capabilities. For some materials that do have high crystallization rates (e.g. Te-Sn alloys, the data retention times are often not adequate because of the instability of the amorphous state.
Thus, the problem was that the prior art had not provided write-once optical recording layers which possess the combination of (a) a crystallization rate less than 1.0 .mu.s, (b) good corrosion resistance, (c) a stable amorphous state and (d) a capability of high rate, high density recordings.
This problem was solved in the first mentioned related application; copending U.S. Ser. No. 014,336 filed 2/13/87. In that application there is disclosed an alloy of antimony-tin and, in preferred embodiments, a third element indium, which alloy is capable of high performance write-once optical recording. The recording materials of that application do not suffer the environmental corrosion seen in chalcogen rich thin films typically used for write-once applications. The rate of crystallization of the antimony-tin optical recording layers is less than 1 .mu.s using practical laser power (&lt;12 mW). The dynamic recording sensitivity at 10 m/s is in the range of 3.5 to 6.5 mW. The amorphous state is very stable, particularly in those embodiments where the alloy includes indium. Thus, recordings on the thin film are made using the amorphous to crystalline transition mechanism. The layers are capable of high density, high rate recordings having a dynamic carrier-to-noise ratio (CNR) over 55 decibels, particularly in the range of 60 to 65 decibels.
The superior properties of these alloys are believed to be a result of the NaCl (or slightly distorted NaCl) type crystalline structure of the antimony-tin intermetallic phase. It is believed that this structure facilitates the fast transformation from the amorphous phase. While the binary antimony-tin alloy performs better than prior art materials, it still has a relatively low crystallization temperature and hence, is not suitable for applications where severe temperature conditions are to be expected. Further, the carrier to noise ratio is about 55 dB for the binary alloy. This is adequate for most applications but improvement is desirable for more demanding ones.
Indium can be used to stabilize the amorphous phase by increasing the amorphous to crystalline transition temperature. Indium was first selected since its atomic number (49) is similar to that of antimony (51) and tin (50). The use of indium also results in a significant improvement in the carrier-to-noise ratio of the recording process.
Subsequent to the discovery that indium could be used to improve the properties of the basic antimony-tin alloy, other basically antimony-tin alloys were discovered by the present assignee. Thus, applications were filed on antimony-tin alloys containing aluminum (U.S. Ser. No. 058,721 filed June 5, 1987); zinc (U.S. Ser. No. 058,722 filed June 6, 1987); and germanium (U.S. Ser. No. 014,337 filed Feb. 13, 1987).
In Japanese published patent application number J62-246,788, published Oct. 27, 1987 there is disclosed an antimony-tin-germanium alloy for optical recording. There is no suggestion in this publication that any other third element could be used with antimony-tin.
A number of other publications disclose a variety of alloys that are proposed for optical recording. However, none of these publications specifically disclose antimony-tin alloys of the present type, that is, alloys that have the advantageous combination of properties described above. Representative references are: U.S. Pat. No. 4,686,543 to Tani et al; U.S. Pat. No. 4,405,706 to Takahashi et al; 4,357,616 to Terao et al; 4,230,939 to deBont et al; 4,647,944 to Gravesteijn; and Japanese published applications numbers J60-177,446 and J58-7,394.